The plan, entitled the Tax Cuts and Jobs Act (TCJA), changes individual income tax rates, lowering the top rate from 39.6 percent to 38.5 percent, while creating another tax bracket.

Taxes are decreased for everyone except the lowest earners, which remains at 10 percent for those who make between $0 and $19,050 a year.

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TCJA also cuts the corporate tax rate from 35 percent to 20 percent, beginning in 2019.

According to the nonpartisan Joint Committee on Taxation, the reforms will add $1 trillion to the already sizable federal budget deficit in the US.

Republicans have long been known as "budget hawks", voting against legislation that would add to the budget deficit. Four Republican senators held out against the bill due to, among other reasons, fears of a budget deficit increase.

An increase in the budget deficit would add to the US national debt, which currently stands around $20 trillion.

Senate Minority Leader Nancy Pelosi said in a statement on Thursday that "Republicans are in full-blown delusion about the true consequences of their monstrous bill".

Republican Senator Jeff Flake of Arizona, one of the four holdouts, announced he would support the bill on Friday afternoon, giving Republicans 50 votes in favour, with 50 against.

"From the outset of the current debate on tax reform, my goal has been to ensure that Congress passes a tax reform package that is both fiscally-responsible and promotes economic growth," Flake said in a statement.

The lower tax rate for corporations was a persuading factor for Flake, who said that for "nearly two decades I have advocated for a lower corporate tax rate" that allows for US corporations to "compete globally".

Flake also tweeted that Senate leaders have agreed to enact "fair" and "permanent protections" for Deferred Action for Childhood Arrivals (DACA) recipients, also known as DREAMers.

DACA recipients are undocumented people living in the US who were brought there by their parents in their youth.

The centre-left Center for American Progress produced a study in August 2017 that showed 91 percent of DACA recipients were employed and made an average wage of $17.46 hourly, well above the US minimum wage of $7.25.

President Trump ended DACA in October and attempted to use recipients as leverage for stricter border security, including funding for his proposed border wall with Mexico.

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If the vote tally doesn't change, Vice President Mike Pence could, in his role as president of the Senate, cast the deciding vote. The move would pass tax reform.

The passing of TCJA would give Republicans and Trump a victory after failed attempts to repeal former President Barack Obama's landmark healthcare reform.

Republicans in the Senate were hoping to pass the bill this week so they could agree on final language for the tax overhaul with the House of Representatives, which they also control, later this month.